This application is based on the application No. 2002-12701 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image taking apparatus such as a compact camera, a movie camera, a digital camera and a surveillance camera having a distance measuring device according to an external light passive auto focus (AF) method.
2. Description of the Related Art
In the subject brightness, there is a light quantity difference of approximately one hundred thousand times between sunlight in the daytime and moonlight in the nighttime. In order that appropriate distance measurement and exposure can be obtained even under taking conditions where there is such a light quantity difference, conventionally, a distance measuring device is used that is provided with a brightness monitor and uses image sensors which are an application of photoelectric conversion.
In this distance measuring device, an image sensor for the brightness monitor placed close to an image sensor for distance measurement is provided, and in accordance with the output from the image sensor for the brightness monitor, the accumulation time (integration time) of charges generated by the image sensor for distance measurement is controlled.
However, since the subject brightness distribution is not uniform over the entire area of the image sensor and dark parts and bright parts are intermingled with each other, when a brightness monitor monitoring the entire area of the image sensor for distance measurement is used, bright parts and darks parts exceed the dynamic range of the image sensor, so that it is impossible to perform control for sufficiently obtaining the contrast of the subject.
That is, conventional distance measuring devices are typically provided with a brightness monitor 73 detecting the brightness of the subject and being placed close to, of a pair of left and right distance measurement image sensor arrays 71 (71L, 71R), the integration-type image sensor array 71L as shown in FIG. 12. An integration control circuit 74 outputs a signal to start accumulation of the charges generated by the integration-type image sensor arrays 71 when receiving an integration start signal, and terminates the accumulation of the charges generated by the image sensor arrays 71 when receiving from an integration automatic termination determiner 75 a signal representative of automatic termination of the integration determined in accordance with the typical brightness of the subject. That is, the brightness monitor 73 transmits a signal representative of the typical brightness of the subject to the integration automatic termination determiner 75, and the integration control circuit 74 controls the image sensor arrays 71 so as to determine the charge accumulation time based on the value of the brightness.
However, since the optical system of the conventional distance measuring device is not interlocked with the taking optical system of the camera, when the taking area is changed by a change in the focal length of the taking optical system or the like, the measurement area of the brightness monitor is not interlocked with the change. That is, under the control of a microcomputer 76, the focal length of the taking optical system 78 is changed by the zoom lens system being moved by a motor 77, so that the taking area is changed. On the other hand, the optical system 72 of the distance measuring device has no mechanism to change its focal length in response to a movement of the taking optical system. Therefore, when the focal length of the taking optical system is short and the image plane (taking area) of the taking optical system is large, the image sensor arrays 71 or the brightness monitor 73 is included in the taking area as shown in FIG. 13(a). In this case, even when a high-brightness light source 81 such as the sun is present in the background of a main subject 80, in an output signal 21a from the brightness monitor 73, the brightness of the part of the subject 80 is high.
When the taking area of the taking optical system is decreased for a reason such that the focal length of the taking optical system 78 is increased, there are cases where parts of the image sensor arrays 71 or the brightness monitor 73 of the distance measuring device extend off the image plane of the taking optical system as shown in FIG. 13(b). In such cases, the brightness monitor 73 is situated so as to overlap with the high-brightness light source 81 present in the background of the main subject 80. Therefore, in these cases, in an output signal 21b from the brightness monitor 73, the brightness of the high-brightness subject 81 is high, and the charge accumulation time of the image sensor arrays 71 is determined by using as the reference (typical brightness) the brightness of the light source 81 not included in the image plane of the taking optical system. Consequently, a typical brightness insufficient for obtaining an image of the main subject 80 is output from the brightness monitor 73, so that the determined integration time is not the one for sufficiently obtaining the brightness distribution of the main subject 80.
Accordingly, an object of the present invention is to provide a camera having a distance measuring device according to the external light passive AF method capable of responding to changes in taking magnification and not readily affected by a light source in the background.
To attain the above-mentioned object, the present invention provides an image taking apparatus comprising: a taking optical system for imaging a subject light; a distance measurement optical system having an optical path different from an optical path of the taking optical system, and not interlocking with a change of a focal length of the taking optical system; a pair of image sensor arrays for receiving subject light imaged by the distance measurement optical system; a plurality of brightness monitors being placed close to one of the image sensor arrays, each independently for measuring a brightness of a subject in a predetermined area and for outputting a signal corresponding to the brightness; a selector for selecting a predetermined brightness monitor from among the brightness monitors based on a image taking condition; and a controller for controlling integration by the image sensor arrays based on the signal from the brightness monitor selected by the selector, wherein one of the brightness monitors has a measurement area being large compared to an other brightness monitor, and covers a measurement area of the other brightness monitor.
According to this structure, since a plurality of brightness monitors having different measurement area is provided, the brightness of the subject can be measured in different area. Therefore, for example, when the taking area is decreased because of a change of the focal length of the taking optical system, by using a brightness monitor having a small measurement area, measurement is never affected by the brightness of a subject other than the main subject. Consequently, the typical brightness based on the brightness of the main subject can precisely be measured. Moreover, since the brightness monitors each independently measure the brightness of the subject and one of the brightness monitors is selected, the control of the integration time is short compared to a case where each brightness monitor outputs a brightness signal and processing is performed one after another.
In the following description, like parts are designated by like reference numbers throughout the several drawings.